Prosthetic facet joint ligament

ABSTRACT

This invention relates to a spinal facet joint ligament.

BACKGROUND OF THE INVENTION

One of the most common surgical interventions today is arthrodesis, orspine fusion, in which two or more adjacent vertebral bodies are fusedtogether in order to alleviate pain associated with the disc(s) locatedbetween those vertebral bodies. Approximately 300,000 such proceduresare performed annually in the United States alone. Clinical successvaries considerably, depending upon technique and indications, andconsideration must be given to the concomitant risks and complications.

While spine fusion generally helps to eliminate certain types of pain,it has also been shown to decrease function by limiting the range ofmotion for patients in flexion, extension, rotation and lateral bending.Furthermore, it is believed that spine fusion creates increased stresseson (and, therefore, accelerated degeneration of) adjacent non-fusedmotion segments. Additionally, pseudoarthrosis, resulting from anincomplete or ineffective fusion, may reduce or even totally eliminatethe desired pain relief for the patient. Also, the fusion device(s) usedto effect fusion, whether artificial or biological, may migrate out ofthe fusion site, thereby creating significant new problems for thepatient. Lastly, the recuperation time after a fusion procedure can belengthy.

Recently, several attempts have been made to recreate the naturalbiomechanics of the spine through the use of an artificial disc.Artificial discs are intended to restore articulation between vertebralbodies so as to recreate the full range of motion normally allowed bythe elastic properties of the natural disc, which directly connects twoopposed vertebral bodies. However, the artificial discs developed todate do not adequately address the mechanics of motion of the spinalcolumn.

In addition to the foregoing, posterior elements called the facet jointshelp to support axial, torsional and shear loads that act on the spinalcolumn. Furthermore, the facet joints are diarthroidal joints thatprovide both sliding articulation and load transmission features. Thefacet's articular surfaces contact in extension, limiting rotation andincreasing compressive load. The articular surfaces also contact on oneside of the spine in lateral bending and axial rotation, also limitingrotation and transferring load.

However, the facet joints can also be a significant source of spinaldisorders and, in many cases, debilitating pain. The articularcartilaginous surfaces can degenerate due to mechanical or biologicalfactors and cause pain as with other joint osteoarthritis. For example,a patient may suffer from arthritic facet joints, severe facet jointtropism or otherwise deformed facet joints, facet joint injuries, etc.There is currently a lack of suitable intervention procedures for facetjoint disorders. Facetectomy, or the removal of the facet joints, mayprovide some relief, but is also believed to significantly decrease thestiffness of the spinal column (i.e., hypermobility) in all planes ofmotion: flexion and extension, lateral bending, and rotation.Furthermore, problems with the facet joints can also complicatetreatments associated with other portions of the spine. By way ofexample, contraindications for artificial discs include arthritic facetjoints, absent facet joints, severe facet joint tropism or otherwisedeformed facet joints. Accordingly, there is a need for a facet jointreplacement that addresses these concerns.

U.S. Pat. No. Re. 36,758 (Fitz I) discloses an artificial facet jointwhere the inferior facet, the mating superior facet, or both, are simplycovered with a cap. Because placement of the cap requires no preparationof the bone or articular surfaces; it covers and, therefore, preservesthe bony and articular structures.

However, simple capping of the facet has several potentialdisadvantages. If the facet joint is osteoarthritic, a cap will notremove the source of the pain. Additionally, at least in the case ofsurface replacements for osteoarthritic femoral heads, the capping ofarticular bone ends has proven to lead to clinical failure due tomechanical loosening. This clinical failure is hypothesized to be aconsequence of disrupting the periosteum and ligamentum teres femoris,both of which play a role in delivering nutrition to the femoral head,thereby leading to avascular necrosis of the bony support structure forthe surface replacement. It is possible that corresponding problemscould develop from capping the facet. Another potential disadvantage offacet capping is that in order to accommodate the wide variability inanatomical morphology of the facets, not only between individuals butalso between levels within the spinal column, as well as due toassociated hypertrophic and degenerative changes, a very wide range ofcap sizes and shapes is required.

U.S. Pat. No. 6,280,444 (“Zuchermann”) describes a spacer that is wedgedbetween the spinous processes, a spinal extension stop, and methods toimplant it. According to Zuchermann, the spacer limits rotation inextension (?) while providing flexion mobility. The U.S. Pat. No.6,280,444 described a method to reduce stenosis by increasing posteriordisc height and limiting extension. However, it is unknown exactly whatfunction such a device would have. This device may abnormally loads thespinous processes and therefore could lead to abnormal bone remodeling.It affects the posterior elements and provides mobility, but does notalleviate posterior structures or address facet syndromes.

U.S. Pat. No. 6,132,464 (“Martin”) describes a replacement of thearticular surfaces and means for supporting and fixing thesereplacements to the posterior processes. The articulating surface itselfis described as having “the shape, position, and orientation of anatural articular facet”. It discloses a spinal facet joint prosthesisthat is supported on the lamina (which is sometimes also referred to asthe posterior arch). Extending from this support structure are inferiorand/or superior blades that replace the cartilage at the facet joint.The prosthesis of U.S. Pat. No. 6,132,464 generally preserves existingbony structures and therefore does not address pathologies which affectthe bone of the facets in addition to affecting the associatedcartilage. Furthermore, the prosthesis of U.S. Pat. No. 6,132,464requires a secure mating between the prosthesis and the lamina. However,the lamina is a very complex and highly variable anatomical surface. Asa result, in practice, it is very difficult to design a prosthesis thatprovides reproducible positioning against the lamina so as to correctlylocate the cartilage-replacing blades for the facet joints.

The U.S. Pat. No. 6,132,464 describes articular surfaces and means ofattachment, but does not describe a capsular replacement.

U.S. Pat. No. 5,571,191 (“Fitz II”) describes a facet prosthesiscomprising superior and inferior components, pyramidal or conical inshape, fitting over the facet processes, and having low friction matingsurfaces. Although this patent describes articular surfaces and means ofattachment, it does not describe a capsular replacement.

Gardner et al. Eur. Spine J (2002) (Supp 2): S157–163, discloses Grafligamentoplasty as a means of stabilizing and reducing mobility of oneor more severely symptomatic motion segments associated withdegenerative disc disease. FIG. 1 shows Polyester bands wrapped around apair of pedicle screws extending from adjacent vertebral bodies.According to Gardner, appropriate Graf bands immobilizes the motionsegment in lordosis with the facet joints in a position of fullextension, in which position they are very stable. See page S159.Accordingly, Graf ligamentoplasty essentially immobilizes the facetjoint. Gardner does not disclose a ligament that traverses a facetjoint.

Senegas et al., Eur. Spine J. (2002) 11 (Supp 2): S164–9 discloses aWallis implant system comprising a titanium interspinous blocker and aDacron ligament, wherein the blocker is placed between two spinousprocesses and the Dacron ligament wraps around spinous processes. See p.S165. Accordingly, Senegas does not disclose a ligament that traverses afacet joint.

WIPO PCT Published patent application No. WO 00/53126 (“Ogun”) disclosesa memory metal implant for fixing an articulated joint, including afacet joint.

The Dynasis system is generally used as a replacement for the naturalposterior longitudinal ligament. The system includes a cable housedinside a plastic sheath, and is attached to superior and inferiorpedicles. The ligament of the Dynasis system does not traverse a facetjoint.

SUMMARY OF THE INVENTION

The present inventors have appreciated that natural facet joints aretrue articulating joints in which the capsule surrounding the articularsurfaces play a very important role. While the articular surface of thejoint transfers compression, the capsule transfers tension. In flexion,the joint opens and the capsule is stretched. Several biomechanical invitro studies have demonstrated the contribution of the capsule to totalmotion segment stiffness in flexion.

Replacing the articular surface may relieve pain, but does not fullyrestore joint functionality.

Accordingly, the present inventors recognized a need for stabilizing thefacet joint in both compression and tension, and for preventingproliferation of debris from the implant site to the surroundingtissues.

In one aspect of the present invention, the facet joint is stabilized inboth compression and tension by a prosthetic ligament having fastenersfixated either in the superior and inferior vertebrae or in superior andinferior prosthetic facet joint components. In some embodiments, thefasteners are selected from the group consisting of bone screws, hooks,wires, and pins. In some embodiments, the intermediate portion of theligament is selected from the group consisting of a cable, a wires, aninterconnected face, and a soft polymer bonded to the fastener andstretching between the superior and inferior fastener.

Accordingly, the present invention replaces the natural facet jointcapsule with an. artificial construct that more fully provides thenatural mechanical relationship provided by a natural healthy facetjoint. In particular, by providing a ligament that stretches whileresisting tension increases joint stability, the present invention moreclosely simulates physiological contributions of the facet joint capsuleand so more closely approximates a full natural facet joint.

In one aspect of the present invention there is provided a prostheticfacet joint ligament. In preferred embodiments, this prosthetic facetjoint ligament can be attached to anchoring points on opposing sides ofa natural or prosthetic facet joint to provide a constraint againstrelative movement of the facet joints.

In a preferred embodiment of the present invention, the ligament isshaped as a sheath that can prevent debris produced by the facetarticulation from spreading to the surrounding tissues, in particular tovarious neural structures. Previous facet joint replacement inventionsdescribe resurfacing techniques that replace the contacting faces of thefacet joint with metals or polymers. Due to unique variation in motionsof the facet joint, these resurfaced contacting faces will inevitablyproduce wear debris, which is likely to irritate tissues. A membrane orsheath that surrounds the contacting faces and captures generatedparticles can reduce tissue irritation and inflammation. The membrane orsheath may also have structural integrity in itself and resistover-stretching and thereby supply resistance to tension.

In some embodiments, the prosthetic facet joint ligament is used incombination with other prosthetic facet joint components to produce aprosthetic facet joint having both load-bearing andmovement-constraining functions.

Therefore, in accordance with the present invention, there is provided aprosthetic facet joint ligament.

In some embodiments, the prosthetic facet joint is used in combinationwith intervertebral body components to produce a total vertebralsolution addressing both anterior and posterior loading and movementissues.

Therefore, in accordance with the present invention, there is provided afacet joint prosthesis for replacing a natural facet joint comprisingfirst and second facets, the prosthesis comprising:

-   -   a) a superior facet joint component having a superior outer        surface adapted to attach to a superior facet,    -   b) an inferior facet joint component having an inferior outer        surface adapted to attach to an inferior facet, and    -   c) a ligament adapted to constrain relative movement between the        facets.

DESCRIPTION OF THE FIGURES

FIG. 1 discloses a pair of facet joint ligaments of the presentinvention attached across a pair of facet joints of a functional spinalunit.

FIG. 2 discloses a pair of facet joint ligament of the presentinvention, wherein each ligament is a capsule, respectively, attachedacross a facet joint of the same functional spinal unit.

FIG. 3 discloses preferred embodiments of a facet joint ligament of thepresent invention.

FIG. 4 discloses a driver used to attach the facet joint ligament ofFIG. 3

FIG. 5 discloses an embodiment of the ligament of the present inventioncomprising a curved, flexible intermediate portion attached across afacet joint.

FIG. 6 discloses an embodiment of the ligament of the present inventioncomprising a spring intermediate portion.

FIG. 7 discloses an embodiment of the ligament of the present inventioncomprising a a pair of curved, flexible intermediate portionsencapsulated in a polymer.

FIG. 8 discloses an embodiment of the ligament of the present inventioncomprising a a pair of straight intermediate portions attached by apolymer

FIG. 9 discloses superior and inferior prosthetic facets having opposedarticulating surfaces.

FIG. 10 discloses superior and inferior facets having a prostheticcushion attached therebetween.

FIG. 11 discloses a pair of facet joint ligament of the presentinvention, wherein the ligament is a capsule, attached across a pair offacet joints of a functional spinal unit having a fusion cage insertedtherein.

FIG. 12 discloses a pair of facet joint ligament of the presentinvention, wherein the ligament is a capsule, attached across a pair offacet joints of a functional spinal unit having a three-piece motiondisc inserted therein.

FIG. 13 discloses a pair of facet joint ligament of the presentinvention, wherein the ligament is a capsule, attached across a pair offacet joints of a functional spinal unit having a two-piece motion discinserted therein.

FIG. 14 discloses a pair of facet joint ligament of the presentinvention, wherein the ligament is a capsule, attached across a pair offacet joints of a functional spinal unit having a cushion-type motiondisc inserted therein.

FIG. 15 discloses an embodiment of the ligament of the present inventionattached across a pair of spinous processes coupled with a prostheticfacet joint.

FIG. 16 discloses an embodiment of the ligament of the present inventionwrapped around a facet joint.

DETAILED DESCRIPTION OF THE INVENTION

Now referring to FIG. 1, there is provided a first embodiment of thefacet joint ligament 101 of the present invention. The ligamentcomprises first 103 and second 105 attachment end portions and anintermediate portion 107. The first attachment end portions attaches tothe superior facet SF of the facet joint, while the second attachmentend portion attaches to the inferior facet IF of the facet joint. Thus,the ligament traverses the facet joint.

Now referring to FIG. 2, there is provided a second embodiment of thefacet joint ligament of the present invention, wherein the ligament is acapsule 111 surrounding the facet joint The capsule comprises first 113and second 115 attachment end portions and a sheath portion 117. Thefirst attachment end portions attaches to the superior facet SF of thefacet joint, while the second attachment end portion attaches to theinferior facet IF of the facet joint. The sheath traverses andcompletely circumscribes the facet joint. In this FIG. 2, the sheath maycover a portion of the exposed bony surface.

The ligament of the present invention can be made of any biocompatiblematerial adapted for constraining but not totally eliminating relativemovement between facet joints. In this regard, the facet joint ligamentof the present invention mimics the natural facet joint capsule. Theligament of the present invention comprises three features. First itmust be adapted to traverse a facet joint. Second, it must allow someflexion to occur across the facet joint. Third, it must resist excessiveflexion of the facet joint.

In preferred embodiments, the ligament comprises a pair of attachmentend portions and an intermediate portion.

Each attachment end portion of the ligament is adapted to attach to ananchoring surface on opposite sides of the facet joint. Typically, theattachment end portion comprises a fastener. In other cases, attachmentmay be provided by sutres or biologically compatible glues. However, inother embodiments, an attachment end portion can simply be terminusbeing identical in design to the intermediate portion. In such a case,the terminus is inserted into a port located on the anchoring surface,such as a port on a prosthetic having a facet joint articulatingsurface.

The intermediate portion of the ligament may be adapted to havedesirable mechanical qualities found in ligaments, such as elasticity,flexibility, tensionability, and extensibility. Combinations of thesequalities allows some displacement of the articular surfaces, butresists excessive displacement.

Preferably, the intermediate portion of the facet joint ligament is madefrom a nonbioresorbable material including polyesters, (particularlyaromatic esters such as polyalkylene terephthalates, polyamides;polyalkenes; poly(vinyl fluoride); polyurethanes;polytetrafluoroethylene (PTFE); carbon fibres; silk; rubber, hydrogels,and glass, and mixtures thereof.

Preferably, the intermediate portion of the facet joint ligament isprovided as a fabric. The fabric may be formed by a flat or circularweaving, knitting, braiding, crocheting or embroidery. Preferably, thefabric is braided in order to provide a high tensile strength. Preferredmaterials suitable for use as fabrics include polyester, polypropylene,polyethylene, carbon fiber, glass, glass fiber, polyurethane,polyaramide, metals, polymers, copolymers, polyactic acid (PLA),polyglycolic acid (PGA), silk, cellusoseic acid, and polycaprolactonefibers.

It is anticipated that, in use, the intermediate portion of the facetjoint ligament may rub against soft tissue structures and damage notonly those structures but itself as well. Therefore, in someembodiments, the intermediate portion of the facet joint ligament islubricated. The lubricants lowers the friction coefficient between theligament and the soft tissue, thereby lowering the wear. Preferredlubricants include hyaluronic acid, proteoglycans, and hydrogels

In some embodiments, the ligament comprises a material havingorthobiologic properties. This material will help the body'sregenerative processes regrow a natural ligament to replace theprosthetic ligament of the present invention.

In some embodiments, the ligament comprises an orthoconductive portion.The orthoconductive portion typically has a porosity (preferably betweenabout 20 μm and 250 μm) that is adapted to allow the ingress of theosteoconductive cells and an internal surface defined by the porositythat is adapted to attach these cells. In some embodiments, theorthoconductive portion comprises subintestinal submucosa (SIS).

In some embodiments, the ligament comprises an orthoinductive portion.The orthoinductive portion is preferably a protein, and is morepreferably a growth factor. Preferred growth factors include factorsfrom the TGF-beta and IGF-families.

In some embodiments, the ligament comprises an orthogenetic portion. Theorthogenetic portion preferably comprises mesenchymal stem cells. Morepreferably, the MSCs are present in a concentration greater than thatpresent in the patient's natural bone marrow.

In some embodiments, only the intermediate portion of the ligamentcomprises an orthobiologic material. In some embodiments, only theattachment end portion of the ligament comprises an orthobiologicmaterial. In other embodiments, each of the intermediate and atachmentend portions of the ligament comprises an orthobiologic material.

Preferably, the ligament is provided in a sterile form. In someembodiments, the ligment is sterilized, and then placed in a package.Preferably, the inside surface of the package is also sterile.

In some embodiments, the intermediate portion of the ligament istensionable. A tensionable ligament sags when the ends of the ligamentsare moved sufficiently closed to one another so that length of theligament is less the distance between its ends. This quality allows theopposing facets to move closer to each other under loads withoutresistance from the ligament. A tensionable ligament also becomes tautwhen its ends are moved sufficiently away from one another so thatlength of the ligament is about equal to the distance between its ends.This quality constrains relative movement between the opposing facets.

In some embodiments, the tensibility of the ligament is between 10 and50 N/mm.

In other embodiments, the ligament is not tensionable.

In some embodiments, at least a portion of the intermediate portion ofthe ligament is extensible. An extensible ligament has a first at-restlength when its ends are not loaded, and a second larger length when theligament is subjected to tensioning. This quality allows the ligament to“give” a predetermined amount under tension. This quality isadvantageous because the natural facet joint ligament is alsoextensible. Preferably, the ligament has an extensibility of between 45%and 65% of the at rest length of the ligament when subjected to a loadof about 250 N.

In some embodiments, the extensibility of the ligament is between 30 and70 N/mm.

In other embodiments, the ligament is not extensible.

In some embodiments, at least a portion of the intermediate portion ofthe ligament is flexible. A flexible ligament bows under axialloading/easily bends under physiologic flexural loading and easilyregains its shape when the loading is ceased. This quality allows theligament to “give” a predetermined amount while transferring stressunder axial loading. This quality is advantageous because the naturalfacet joint ligament is also flexible. Preferably, the flexible portionof the ligament comprises a curved portion.

Now referring to FIGS. 5–8, there are provided some preferredembodiments of the ligament invention.

Now referring to FIG. 5, in some embodiments, at least a portion of theligament comprises a curved, flexible intermediate portion 61.

Now referring to FIG. 6 in some embodiments, at least a portion of theligament comprises a spring 65. The spring quality allows the ligamentto initially yield to and eventually resist an axial compressive ortension load. In some embodiments, the spring is an expansion spring. Inother embodiments, the spring is a compression spring.

In some embodiments, the intermediate portion of the ligament comprisesa first 71 and second 73 intermediate portions bonded together. Forexample, now referring to FIG. 7, there is provided an embodiment of theligament of the present invention comprising a pair of curved, flexibleintermediate portions encapsulated in an encapsulant 75 (such as apolymer), wherein the ligament is adapted to attach across a facetjoint. The advantage of this embodiment is its ability to provideelastic resistance to loads in a plurality of directions and its abilityto resist substantial tensile forces due to interlocking members.

Now referring to FIG. 8, there is provided an embodiment of the ligamentof the present invention comprising a pair of straight intermediateportions 81,83 attached by a bonding material 85 (such as a polymer),wherein the ligament is attached across a facet joint. The advantage ofthis embodiment is its simple design and small size.

In embodiments to be used in the lumbar spine, the ligament of thepresent invention preferably has a length of between about 1 cm andabout 4 cm. It preferably has a width of between about 0.5 cm and about1 cm.

In some embodiments, the ligament comprises a sheath. In theseembodiments, the width of the sheath is much greater. In preferredsheaths, the sheath is sized to substantially enclose the facet joint.In some embodiments, the sheath is fluid permeable. This feature permitsthe ingress of fluids that help lubricate the joint, while preventingthe egress of wear debris from the facet joint articulation surfaces. Insome embodiments, the sheath contains a lubricating fluid, therebyimitating a natural facet joint capsule. In preferred embodiments, thesheath may be pre-assembled prior to implantation, or it may be attachedvia glues, sutres, wires, thermally activated coagulation or in situpolymer embedding.

As noted above, in some embodiments, the attachment end portions of thefacet joint ligament comprise a pair of fasteners. The function of thefastener is to join to attachment surfaces located on either side of thefacet joint in order to securely fasten the intermediate portion of thefacet joint ligament across the facet joint. The fastener may be adaptedto fasten the facet joint ligament to attachment surfaces located uponeither:

a) a facet joint prosthetic component, or

b) a bony surface located adjacent the facet joint prosthetic component.

The attachment end portions of the prosthetic ligament of the presentinvention may be attached to any two anchoring surfaces on oppositesides of the facet joint, provided the ligament traverses the facetjoint . These anchoring surfaces may be located on a bony surface of theopposing vertebrae, or on other prosthetic facet joint components.

In one embodiment, the first attachment end portion of the ligament isadapted to attach to a first load-bearing portion of a facet jointprosthesis. This embodiment is surgeon friendly in that the attachmentcan be made by the manufacturer prior to surgery, thereby providing easeof use and repeatability.

In another embodiment, first attachment end portion of the ligament isadapted to attach to a portion of the natural vertebra. In someembodiments thereof, the vertebral body is used as the anchoringsurface. In another, the facet portion of the vertebra is the anchoringsurface. In other, and now referring to FIG. 15, the spinous process isused as an anchoring surface.

In another embodiments, the first end portion of the ligament is adaptedto attach to the transverse process. In another embodiments, the firstend portion of the ligament is adapted to attach to the pedicle. Inanother embodiments, and now referring to FIG. 16, the ligament iswrapped around the facet joint.

The fastener may be any design known in the art, including winged,barbed or screw-in mechanisms. Preferably, the fastener is a barbedanchor, as it prevents pullout and is easily inserted. When theattachment surface is a bony surface, the fastener may be a bonefastener.

Preferably, the ligament and fastener components are pre-connected. Thatis, the components are physically attached prior to their placement uponthe spine. Pre-connected components are advantageous because theirsecured attachment to each other are guaranteed, and the surgeon neednot waste time in making the attachment. Components may be pre-connectedby physical locking, physical connection (as in FIG. 3) by bonding, orby making the components from the same material and integrallyconnecting them. When the preconnected components are integrally formed(by, for example, molding or thermoforming), there is no danger ofmicromotion. Therefore, in accordance with the present invention, thereis provided a facet joint ligament comprising:

-   a) a ligament comprising first and second end portions, and-   b) first and second fasteners,    wherein the first fastener is pre-connected to the first end portion    of the ligament, and the second fastener is pre-connected to the    second end portion of the ligament.

Now referring to FIGS. 3 a and 3 b, there is provided a facet jointligament 3 having:

i) an intermediate portion 5,

ii) first and second end portions 7,9, and

iii) first and second conformable portions 11, 13,

wherein the first conformable portion is disposed between theintermediate portion and the first end portion, and the secondconformable portion is disposed between the intermediate portion and thesecond end portion, and

iv) first and second fasteners 15,17,

wherein the first end portion 7 is shaped to cooperatively connect tothe first fastener 15, and the second end portion 9 is shaped tocooperatively connect to the second fastener 17.

Now referring to FIG. 3 c, preferably, the fastener 19 comprises alongitudinal shank 21, an insertion end 23 comprising protrusions 25laterally extending from the shank, and an attachment end 27 having anupper surface 31 for connecting to the ligament.

Preferably, the lateral protrusions have leading edges 28 which definean angle α of no more than 45 degrees relative to the axis of the shank.In such embodiments, the bearing of the leading edge against thevertebral body surface will not substantially impede the progress of thebone fastener into the bone. Preferably, the leading edges define anangle of no more than 30 degrees, and more preferably between about 20degrees and 30 degrees. When the angle α is between 20 and 30 degrees,the angle is sufficiently small to not impede the progress of the bonefastener, and yet sufficiently large to insure its secure fit.

In some embodiments, the height H of the protrusions on the bonefastener is no more than 70% of the diameter D of the longitudinalshank. When this condition is selected, the risk that any protrusionwill act as a shoulder and stop further entry of the bone fastener intothe vertebra is minimized. Preferably, the H/D ratio is no more than40%, more preferably between about 20% and 40%. Within this morepreferred window, the protrusion height is sufficiently small to notimpede the progress of the bone fastener, and yet sufficiently large toinsure its secure fit.

The outer diameter (2H+D) of the bone fastener is preferably betweenabout 3–9 mm, more preferably about 4–6 mm. The length L_(BF) of thebone fastener is preferably between about 3–45 mm, more preferablybetween about 15–25 mm.

In some embodiments, the attachment end of the bone fastener is made ofa ceramic material. When the bone fastener is ceramic, it can withstandthe high impact of the driver without failing. Therefore, in accordancewith the present invention, there is provided an facet joint ligamentcomprising:

-   a) a compressible intermediate portion comprising first and second    end portions, and-   b) first and second fasteners, each fastener having an attachment    end comprising a ceramic material and a shank comprising a polymer    material.

In some embodiments, at least the end portions of the intermediateportion and the attachment end of the bone fastener are made of the samematerial. When the materials are so selected, these portions may beeasily made and pre-connected in an integral fashion. This feature alsoeliminates the need for sutures.

Referring back to FIG. 3 c, in another aspect of the present invention,the attachment end 27 of the fastener is configured to accept a driverelement. When this configuration is selected, the bone fastener may bedriven into the bone by simply providing axial force to the uppersurface 31 of the fastener through the driver. Therefore, in accordancewith the present invention, there is provided a facet joint ligamentcomprising:

-   a) a ligament comprising first and second end portions, and-   b) first and second fasteners,    wherein the first bone fastener is connected to the first end    portion of the ligament, and the second bone fastener is connected    to the second end portion of the ligament, and wherein the first    bone fastener is configured to accept a driver.

Preferably, the configuration defines a recess 29 upon the upper surface31 of the attachment end 27 of the fastener. This recess 29 isconfigured to accept the driver (not shown).

In some embodiments, the recess 29 of the bone fastener may beconfigured to allow insertion of a rescue screw, thereby allowingretrieval of the bone fastener.

Now referring to FIG. 4, in some embodiments, the system has a port foraccepting a driver 51 which drives the bone fastener into the vertebralbody. In FIG. 4, the port comprises a recess 29. In some embodiments,the end portion 53 of intermediate portion is molded to the uppersurface 31 of the attachment end 27 of the fastener such that aninsertion tip 59 of driver 51 can be inserted into the recess 29 withoutdamaging the ligament. FIG. 4 illustrates such an attachment wherein theend portion 53 of the intermediate portion is attachedsemi-circumferentially to the outer edge 55 of the upper surface 31 ofthe fastener. It is appreciated that other embodiments may functionsimilarly, such as attaching the intermediate portion of the ligament toone half face of the fastener upper surface 31 and modifying the drivershoulder to contact the other half face of the fastener surface.Furthermore, the lower surface 57 of the driver shoulder is able tocontact the upper surface 31 of the bone fastener. Preferably, the lowersurface 52 of the insertion tip 59 is also able to contact the bottom 30of the fastener recess 29. With these features, the driver can be usedto fully seat the fastener in the bony attachment surface and applypretension to the ligament. When the diameter of recess 29 issubstantially equal to the diameter of tip 59, the insertion tip willtemporarily reinforce the fastener during the insertion step.

In some embodiments, the lower surface 30 of the recess is located at adepth such that both the distal end of tip 59 and shoulder 59 willrespectively contact the lower surface 30 (as shown in FIG. 3 c) andupper surface 31 of the recess.

In some embodiments, the ligament of the present invention is adapted toreplace the natural interspinous or intertransverse ligament. In otherembodiments, the ligament of the present invention is adapted to replacethe facet joint capsule.

In many embodiments, the attachment end portions of the ligament of thepresent invention are attached to the anchoring surfaces on oppositesides of the facet joint in order to constrain the relative movement ofthe facets.

As noted above, the facet joints are diarthroidal joints that provideboth sliding articulation and load transmission features. The facet'sarticular surfaces contact in extension, limiting rotation andincreasing compressive load.

Preferably, the facet joint ligament of the present invention will beused in conjunction with other prosthetic components designed to mimicthe load-bearing and preferably the sliding articulation functions ofthe natural facet joint. Therefore, in accordance with the presentinvention, now referring to FIGS. 9 and 10, there is provided a facetjoint prosthesis for replacing a natural facet joint comprising firstand second facets, the prosthesis comprising:

-   -   a) a superior facet joint component 91 having an outer surface        93 adapted to attach to a superior facet,    -   b) an inferior facet joint component 95 having an outer surface        97 adapted to attach to an inferior facet,    -   c) a facet joint ligament 99 adapted to constrain relative        movement between the superior and inferior facets.

In some embodiments, the superior and inferior facet joint componentsdisclosed in Fitz I, Fitz II, Zuchermann and Martin, supra, thespecifications are which are incorporated by reference, are selected.

In embodiments of the present invention comprising a prosthesis havingsuperior and inferior facet joint components, the outer surfaces thereofmay comprise an attachment feature. Preferred attachment features areselected from the group consisting of teeth, keels, spikes, pins, andcombinations thereof. In some embodiments, the outer surface is shapedto conform to the natural facet surface, that is, convex.

Now referring to FIG. 9, in some embodiments of the present invention,the superior and inferior facet joint components of the prosthesis areindependent bodies. In preferred embodiments thereof, the superior facetjoint component forms a superior endplate having an outer surfaceadapted to attach to a first facet and an inner articulation surface,while the inferior facet joint component forms an inferior endplatehaving an outer surface adapted to attach to an inferior facet and aninner articulation surface. In this embodiment, the inner articulationsurfaces are adapted to form an articulation interface. For the purposesof the present invention, this embodiment is called an “articulationprosthesis”.

In some articulation embodiments, the first inner articulation surfaceis convex shaped, while the second inner articulation surface is concaveshaped. This creates a ball and socket joint well known in the art.

In some embodiments, the first and second articulation surfaces areconforming. In others, the first and second articulation surfaces arenon-conforming.

Now referring to FIG. 10, in other embodiments, the superior andinferior facet joint components do not have inner articulation surfaces,but rather are joined by an elastic cushion core. In preferredembodiments thereof, the “cushion-type” prosthesis comprises:

-   -   a) a superior facet joint component 201 forming a superior        endplate having an outer surface 203 adapted to attach to a        superior facet and an inner surface 205,    -   b) an inferior facet joint component 211 forming an inferior        endplate having an outer surface 213 adapted to attach to an        inferior facet and an inner surface 215,    -   c) an elastic core 221 having a superior surface 223 adapted to        attach to the inner surface of the superior facet joint        component and an inferior surface 225 adapted to attach to the        inner surface of the inferior facet joint component.        For the purposes of the present invention, this embodiment is        called a “cushion prosthesis”. In preferred embodiments thereof,        the device comprises an elastomer adapted to elastically        compress during axial loading and relax when the load is lifted.

The superior and inferior facet joint components of the presentinvention may be made from any material appropriate for human surgicalimplantation, including but not limited to all surgically appropriatemetals including titanium, titanium alloy, chrome alloys and stainlesssteel, and non-metallic materials such as carbon fiber materials,resins, plastics and ceramics.

The elastic core may comprise polyurethanes, foamed polyethylene,silicones, rubbers, copolymers or hydrogels. In other embodiments, theelastic core is formed of an orthobiologic material such as hyaluronate,collagen or subintestinal submucosa.

In some embodiments of the present invention having both a pair ofprosthetic facet joint articulating surfaces and a prosthetic facetjoint ligament, the invention limits the natural spinal extension. Inthese embodiments, extension is limited to no more than 7 degrees,preferably no more than 5 degrees. Preferably, the device stiffness isat least 2 Nm/degrees in order to so limit the extension.

In some embodiments of the present invention having both a pair ofprosthetic facet joint articulating surfaces and a prosthetic facetjoint ligament, the invention limits the axial compression of theprosthetic articulating surfaces. In these embodiments, axialcompression is limited to between 0.1 mm and 2 mm, and is preferablybetween 0.5 and 1.5 mm. In this preferred range, the axial compressionis similar to that of the natural healthy facet joint.

In some embodiments of the present invention having a prosthetic facetjoint ligament, the invention resists flexion. In these embodiments,flexion is limited to no more than 15 degrees, and preferably is no morethan 12 degrees. Preferably, the tensile strength of the prostheticcapsule is between 50 and 300 N, is preferably at least 100 N, and ismore preferably at least 200 N.

In some embodiments of the present invention having both a pair ofprosthetic facet joint articulating surfaces and a prosthetic facetjoint ligament, the invention resists axial rotation. In theseembodiments, a pair of devices of the present invention are preferablyused so that each facet joint of a functional spine unit has a device,whereby a first device limits the axial rotation while the ligament ofthe second device is put in tension. This motion tends to producecoupled motion with flexion and bending. In some embodiments, theprosthetic articulating surfaces of the first device are sufficientlystrong to withstand compressive forces of at least 100N, and morepreferably at least 150N, and more preferably at least 200N.

In some embodiments of the present invention having both a pair ofprosthetic facet joint articulating surfaces and a prosthetic facetjoint ligament, the invention resists at least anterior-posterior shear.In these embodiments, the prosthetic articulating surfaces contact andthe prosthetic articulating surfaces are sufficiently strong towithstand contact shear forces of at least 500N, and more preferably atleast 750N, and more preferably at least 1000N.

In some preferred embodiments, at least one prosthetic facet jointcomponent is used in conjunction with an anterior-based intervertebralbody device. Preferably, the at least one prosthetic facet jointcomponent is selected from the group consisting of the superior facetjoint component, the inferior facet joint component, and the facet jointligament, and combinations thereof.

Preferably, the intervertebral body device is selected from the groupconsisting of:

a) an interbody fusion device, and

b) a motion device.

Preferred motion devices are selected from the group consisting ofarticulating devices, cushion devices, nucleus pulposus replacements anddevices comprising a compound adapted to promote nucleus pulposusregeneration (“orthobiologic disc device”).

Preferred interbody fusion devices are disclosed in U.S. Pat. No.4,834,757 (“Brantigan”), and U.S. Pat. No. 5,489,308 (“Kuslich”), thespecifications of which are incorporated by reference.

Now referring to FIG. 11, there is provided:

-   a) an artificial interbody spinal fusion implant 351 for insertion    within an implantation space formed across the height of a disc    space between vertebral bodies of a human spine, the vertebral    bodies having an anterior aspect and a posterior aspect and a depth    therebetween, said implant comprising;    a leading end 353 for insertion first into the disc space and a    trailing end 355 opposite said leading end, said implant having a    length from said leading end to said trailing end;    a top 357 and a bottom 359 between said leading and trailing ends    adapted to space apart the adjacent vertebral bodies, said top and    said bottom each preferably having at least one opening 361    therethrough, said openings being in communication with one another    to permit for the growth of bone from adjacent vertebral body to    adjacent vertebral body through said implant, said implant having a    height H from said top to said bottom;    opposite sides between said top and said bottom, and between said    leading and trailing ends, said implant having a width W from one of    said sides to the other of said sides, the height of said implant    preferably being less than the width of said implant;    said implant being formed at least in part of a material other than    bone; and    said implant being configured to be wholly contained within the    perimeter of the adjacent vertebral bodies, and-   b) a facet joint ligament.

Preferred articulating motion devices are disclosed in U.S. Pat. Nos.5,556,431 and 5,674,296, the specifications of which are incorporated byreference.

Now referring to FIG. 12, in some embodiments, the articulating motiondisc is a three piece design. Thus, the invention comprises:

-   a motion disc comprising:    -   a) a first prosthetic vertebral endplate 371 comprising:        -   i) an outer surface 373 adapted to mate with a first            vertebral body,        -   ii) an inner surface 375 having a first articulation surface            377,        -   iii) a body portion 379 connecting the inner and outer            surfaces,    -   b) a second prosthetic vertebral endplate 381 comprising:        -   i) an outer surface 383 adapted to mate with a second            vertebral body, and        -   ii) an inner surface 385 comprising a first articulation            surface 387,    -   c) a core member 391 comprising:        -   i) a first articulation surface 393 adapted for articulation            with the first articulation surface of the first endplate,            and        -   ii) a second articulation surface 395 adapted for            articulation with the first articulation surface of the            second endplate,            wherein the core member is oriented to produce a first            articulation interface between the first articulation            surface of the first endplate and the first articulation            surface of the core member, and a second articulation            interface between the first articulation surface of the            second endplate and the second articulation surface of the            core member, and-   a facet joint ligament.

Now referring to FIG. 13, in some embodiments, the articulating motiondisc is a two piece design. Thus, the invention comprises:

-   a motion disc 401 comprising:    -   a) a first prosthetic vertebral endplate 431 comprising:        -   i) an outer surface 433 adapted to mate with a first            vertebral body,        -   ii) an inner surface 435 having a first articulation surface            441,        -   iii) a body portion 443 connecting the inner and outer            surfaces,    -   b) a second prosthetic vertebral endplate 411 comprising:        -   i) an outer surface 413 adapted to mate with a second            vertebral body, and        -   ii) an inner surface 415 comprising a second articulation            surface 417,            wherein the first and second articulation surfaces are            oriented produce an articulation interface, and-   a facet joint ligament.

Other preferred articulating motion devices are disclosed in U.S. Pat.Nos. 6,368,350 and 5,507,816, the specifications of which areincorporated by reference.

Preferred cushion motion devices are disclosed in U.S. Pat. Nos.6,136,031; 5,071,437; and 5,645,597, the specifications of which areincorporated by reference.

Now referring to FIG. 14, in some embodiments, the motion disc is acushion disc and the invention comprises:

-   a cushion motion disc comprising:    -   a) a first prosthetic vertebral endplate 451 comprising:        -   i) an outer surface 453 adapted to mate with a first            vertebral body,        -   ii) an inner surface 455,        -   iii) a body portion 453 connecting the inner and outer            surfaces,    -   b) a second prosthetic vertebral endplate 461 comprising:        -   i) an outer surface 463 adapted to mate with a second            vertebral body, and        -   ii) an inner surface 465,    -   c) a core cushion member 471 comprising:        -   i) a first attachment surface 473 adapted for attachment to            the first articulation surface of the first endplate, and        -   ii) a second attachment surface 475 adapted for articulation            with the first articulation surface of the second endplate,            and        -   iii) an elastic body portion 479 connecting the first and            second attachment surfaces of the core cushion member.

Preferred nucleus pulposus replacement devices are disclosed in U.S.Pat. Nos. 5,976,186, 5674,295 and 6,264,695, the specifications of whichare incorporated by reference.

1. A prosthetic facet joint ligament for stabilizing a natural facetjoint, comprising a pair of attachment end portions and an intermediateportion, wherein each attachment end portion of the ligament is adaptedto attach to an anchoring surface on opposite sides of the natural facetjoint, and wherein each attachment end portion comprises a fastener, andwherein the intermediate portion comprises first and second intermediateportions having curved ends, each comprising a nonbioresorbable materialselected from the group consisting of polyesters, polyurethanes;polytetrafluoroethylene (PTFE); carbon fibres; silk; rubber, hydrogels,and glass, and mixtures thereof, wherein the curved ends of the firstand second intermediate portions are interlocking members attached toeach other by a bonding material.
 2. The ligament of claim 1 wherein theintermediate portion comprises an elastic portion.
 3. The ligament ofclaim 2 wherein the elastic portion comprises a polyester.
 4. Theligament of claim 2 wherein the elastic portion comprises rubber.
 5. Theligament of claim 1 wherein the intermediate portion is braided.